The present invention relates to a selective chemical vapor deposition method and a CVD apparatus using the same.
In manufacturing a semiconductor device using a silicon substrate, in order to reduce a contact resistance of an ohmic contact between a wiring metal and a silicon layer doped with an impurity at a high concentration or to suppress a solid phase reaction between the wiring metal and silicon, a method of forming a silicide film between the wiring metal and the silicon layer is attempted. In addition, in order to reduce a sheet resistance of the high-concentration silicon layer, a method of forming a silicide film on the high-concentration silicon layer is attempted. A titanium silicide film having a low resistance is suitable as the silicide film for these purposes.
Examples of a conventional method of forming a silicide film on the surface of silicon are a method in which a silicide film is deposited by sputtering using a silicide target and a method in which a metal film is deposited on the surface of silicon by sputtering using a metal target and the metal and silicon are reacted by annealing to form a silicide film. These sputtering methods, however, have poor step coverage. Therefore, when a pattern size is decreased, a film cannot be uniformly deposited in, e.g., a contact hole. For this reason, instead of the sputtering method, a demand has arisen for a film formation method capable of forming a metal film with good step coverage.
In a CVD (Chemical Vapor Deposition) method, since a source gas is decomposed on the surface of a heated material, a film can be uniformly deposited on even the bottom of a contact hole. As such a CVD method, a blanket CVD method in which a film is uniformly deposited on the entire surface of a substrate is generally used. In recent years, however, a selective CVD method in which a film is deposited on only the surface of a specific material has been developed for several kinds of materials. This selective CVD method can be optimally used to form a titanium silicide film on the surface of a high-concentration silicon layer.
Selective titanium silicide film CVD methods has been reported by Bouteville et al. (Journal of Electrochemical Society: SOLID-STATE SCIENCE AND TECHNOLOGY. Vol. 134, No. 8, (1987) 2080) and Ilderem et al. (Applied Physics Letters, Vol. 53, No. 8, (1988) 688). According to the method reported by Bouteville et al., however, since hydrogen (H.sub.2) gas and titanium tetrachloride (TiCl.sub.4) gas are used as source gases and silicon for titanium silicide is supplied from silicon of a substrate, a titanium silicide film largely consumes the silicon substrate. In addition, this method poses a problem in which a film growth starts with islets, i.e., a problem of so-called significant nucleation. On the other hand, according to the method reported by Ilderem et al., a titanium silicide film is not directly selectively deposited on the surface of silicon, but a polysilicon film is deposited on the entire surface prior to deposition of the titanium silicide film, and then the titanium silicide film is deposited. Polysilicon on an insulating film is etched and removed during deposition of the titanium silicide film. By using this method, although they obtained a thin titanium silicide film having a smooth surface, full selectivity was not obtained but polysilicon or titanium silicide remained on the insulating film. They used TiCl.sub.4 gas and silane (SiH.sub.4) gas as source gases but could not reduce consumption of underlying silicon. According to a similar experiment using the method of Iderem et al. conducted by the present inventors, when a deposition time of a titanium silicide film was prolonged in order to reduce remainder on an insulating film, the titanium silicide film deposited on silicon largely consumes underlying silicon. As a result, the consumption amount became 100% or more the thickness of the titanium silicide film.
Since the selective CVD method obtains the selectivity by a difference between surface properties, film deposition condition is largely affected by the state of the surface of a material for film deposition (a semiconductor or conductor). That is, if cleanness of the surface is poor, properties of selective deposition adversely affect to result in a problem in which no film is deposited, nuclei are deposited only coarsely, or a film having large undulations is deposited. Therefore, it is difficult to deposit a uniform film. Especially when the surface for film deposition is silicon, a native oxide film is immediately grew on the silicon surface if silicon is exposed to an atmosphere containing oxygen or moisture. Therefore, it is difficult to deposit a film with high uniformity and selectivity.
That is, even if a substrate having a silicon surface is dipped in a dilute hydrofluoric acid to remove a native oxide film on the surface and immediately loaded in a CVD apparatus and vacuuming or air substitution is performed, oxidation of the silicon surface cannot be prevented. In addition, even if removal of the oxide film from the silicon surface is performed so that the silicon surface is not exposed to air, the silicon surface is oxidized by a small amount of moisture or oxygen in vacuum or a gas.
Not so many experiments have been conducted for the titanium silicide film CVD method. One reason for this is that no suitable CVD apparatus for this purpose has been developed. In titanium silicide film CVD, TiCl.sub.4 gas can be most conveniently used as a source gas. In order to deposit a titanium silicide film having a low resistance on silicon by using the TiCl.sub.4 gas, however, a silicon substrate must be heated up to a high temperature of 650.degree. C. or more. In addition, a cold wall type apparatus must be used to obtain good selectivity. Furthermore, in order to eliminate a vapor phase reaction and to obtain a proper deposition rate, the pressure of a source gas must be reduced to a low pressure at which the flow of gas becomes a molecular flow.
Since the titanium silicide film is a metal film, an induction furnace or the like cannot be used to heat a substrate. In addition, since the titanium silicide film does not transmit but absorbs light, a method of simple lamp heating or the like cannot be used. Chlorine or a chloride produced upon decomposition of the TiCl.sub.4 gas is a corrosive gas and therefore corrodes a metal or the like. These problems prevent development of an apparatus capable of performing CVD of a titanium silicide film.
The following apparatus can be exemplified as a conventional metal film CVD apparatus. This CVD apparatus is developed mainly for selective CVD of tungsten (W).
In order to perform CVD by heating a wafer temperature up to 400.degree. C. in the conventional cold wall type single wafer processing apparatus, therefore, a CVD method capable of preventing blur on a window which is a drawback of the conventional lamp heating type CVD apparatus is proposed (Japanese Patent Laid-Open No. 63-26366). As shown in FIG. 9, when a wafer 11 held by a susceptor 17 arranged in a reaction chamber 10 is heated by a lamp heater 13 through a window 12 and a source gas 14 is supplied to form a film on the surface of the wafer 11, a purge gas is supplied from a purge gas supply portion 15 provided between the window 12 and the wafer 11 to form a purge gas flow 16. The purge gas flow 16 covers the surface of the purge gas supply portion 15 so that the source gas does not contact the surface of the purge gas supply portion 15 or the window 12, thereby preventing blur on the surface. According to this CVD apparatus and method, a wafer can be heated up to a high temperature in the presence of a source gas to perform CVD with comparatively high reproducibility even by using an apparatus of a cold wall type.
The above CVD method of preventing blur on a window by using a purge gas flow, however, can be used in only a comparatively high pressure region of a gas (source gas+purge gas) in a reaction chamber, in which a gas flow becomes a viscous flow. In a low gas pressure region in which the gas flow becomes a molecular flow, since the source and purge gases are instantaneously mixed with each other, the source gas contacts the surface of the purge gas supply portion 15 or the window 12. Therefore, the surface cannot be protected from being blurred.
Even in a viscous flow region, if a gas having a high adhesion property is contained in the source gas, the purge gas supply portion 15 or the window 12 is gradually blurred by entrance of a small amount of highly adhesive gas. This is because the pressures of the purge and source gases become equal to each other in such a CVD apparatus due to the structure of the apparatus and therefore the purge gas has only a weak effect of preventing entrance of the source gas.
The conventional single wafer processing CVD apparatus, however, cannot manufacture a high-quality thin film with high reproducibility by performing CVD by using a chlorine-based source gas having a high adhesion property and by heating a substrate up to 500.degree. C. or more in a pressure region in which a gas flow becomes a molecular flow. In particular, since film quality of a thin metal film is easily adversely affected by an impurity or crystallinity, it is difficult to stably manufacture, with high reproducibility, a high-quality thin metal film having a composition or a resistance close to that of a bulk by depositing the film from a chlorine-based source gas.